Hi Bob,
Having the horn stop working is not uncommon and the probability is that you will find a displaced or corroded fuse. The horns require high amperage and is the only units serviced by the larger (35 amp US fuse or 50 amp Lucas) of the 2 fuses within the fuse-box. With this in mind and considering that, when activated, all operating power must pass through the horn button switch (located at the top of the steering wheel/column) and a number of connections before reaching the horns, it is not difficult to realize that this is a unit that could be prone to failure. Add infrequent use to this setup and a failure will probably go unnoticed, or even considered, until the horn is actually needed.
Over the years I have installed a number of relays with easily-hidden in-line fuses to offload high amperage units (i.e. head and driving lights, etc.) from their original dependencies (undersized wiring, switches, connectors). Since the horn gains its power after it as passed through the horn-button switch and a number of bullet connections and wiring that is marginal in gauge at best, you may want to consider installing a relay behind the horn in order to provide reliable switchable power more directly to the horn while reducing amperage passing through the horn-button/steering column to only that needed to trigger the relay.
However, since I had installed this invisible alteration some time ago, I must admit I can’t remember the last time I blew the horn. Given this infrequent use, and should you not want to go through the effort to install a relay, I would agree with what others have suggested and, at a minimum, suggested you clean and solder all wiring ends and coat all connectors/connections with dielectric grease.
By the way, I have been told that, in the absence of dielectric grease, Vaseline can be a substitute. Has anyone else heard this?
Good Luck,
Ray (64BJ8P1)